This invention relates to facsimile communications with multiple pairs of gateways in the communication path. More particularly, the present invention relates to detection and avoidance of unnecessary encoding and decoding in relay connections.
In its T.30 and T.4 standards, the Telecommunications Standardization Section of the International Telecommunications Union (ITU-T) has defined procedures for facsimile transmission of documents in a public switched telephone network (PSTN). The T.30 and T.4 standards define a Group 3 facsimile service, also known as Group 3 fax and G3 fax. The entire contents of these standards are incorporated herein by reference. A Group 3 facsimile apparatus includes any one of (1) a facsimile machine, (2) a computer with a facsimile modem and corresponding software, and (3) other similar products.
Fax relay is used to allow Group 3 facsimile transmission between fax terminals where, in addition to the PSTN or ISDN (Integrated Services Digital Network), a portion of the transmission path utilizes low rate digital networks. Fax relay is a technique by which a fax terminal supporting traditional Group 3 facsimile procedures is connected to a gateway emitting a facsimile through a low rate digital network or a packet network to a receiving gateway, which in turn makes a PSTN call to the called Group 3 facsimile equipment. Once the PSTN calls are established on both ends, the two Group 3 terminals are virtually linked. All standard T.30 session establishment and capabilities negotiation is then carried out between the terminals.